A cliff is defined in the Oxford English Dictionary as "a steep rock face, especially at the edge of the sea". Sea cliffs make up long stretches of coast defined as a break in slope between 15 degrees and the vertical, variable in height, found at the contact between the land and sea. They owe their existence in some way to erosion by the sea. Form and height depends on the type of rock - 'hard' rock (metamorphic, igneous and sedimentary rocks) and 'soft' rock (earth cliffs).